Multilevel converters are of interest to use in a number of different power transmission environments. They may for instance be used as voltage source converters in direct current power transmission systems. They are then often used for converting between direct current (DC) and three-phase alternating current (AC). The multilevel converter is then typically based on a number of converter cells, where a cell essentially provides a voltage contribution that is either zero or based on the voltage of an energy storage element of the cell, such as the voltage across a cell capacitor.
The cells are furthermore often placed in phase legs, where there is one phase leg per phase.
One way of controlling the cells of the phase legs is to provide a carrier wave for each cell of each phase leg, where the carrier waves of a phase leg have the same identical shape, typically as a saw-tooth waveform, but displaced in time from each other with a time delay. Each such carrier wave is then compared with a voltage reference which may be shaped as a sine wave. A cell is then switched if the corresponding carrier crosses the voltage reference. This type of control is schematically shown and described in WO 2009/097063.
In such control a situation can occur that the peak value of the carrier wave slightly exceeds the peak value of the voltage reference. If the occurrence of the peak value of the carrier wave occurs in an interval around the peak value of the voltage reference, which interval may be ±30 degrees around the peak value, then the corresponding cell may be switched on almost immediately followed by a switch off or vice versa. This type of switching is normally not necessary, but only leads to unnecessary switching losses. Since the number of cells can be high, the amount of unnecessary switching in a multilevel converter may be significant.
There is therefore a need for limiting the switching.
WO 96/18234 describes one way in which the switching at peak values is limited, which is through so-called dead band pulse width modulation (DBPWM).
In dead band PWM, switching patterns are realized through introducing a common mode voltage, which is basically many zero sequence components with different frequencies, such as 3rd, 5th, 9th, etc. A common mode implies that all three phases have the same waveform shaper. In DBPWM zero sequences are thus added to a voltage reference that is common for a number of phases. This adding of zero sequences raises the voltage in certain areas, such as around the peak voltages, which may be used to avoid switching around peak levels.
JP 2000-69760 and JP 09-149660 also seem to describe variations of DBPWM.
DBPWM is furthermore described by V. G. Agelidis, P. D. Ziogas, G. Joos in “Dead-band PWM switching patterns”, Conf. Rec. IEEE PESC 1992, pp. 427-432.
Also A. M Massoud et al. describe DBPWM in “Control techniques for Multilevel Voltage Source Inverters”, 34th Power Electronics Specialist Conference, Jun. 15-19, 2003, Vol. I, pp. 171-176.
However, the introduction of zero sequences in this way will change the shape of the common voltage reference in other areas than the peak area, for instance at areas where the voltage reference has the steepest slopes. This will lead to unnecessary switching, which is not desirable. In the multilevel converter, the frequency of the carrier is normally much lower than the carrier frequency of a 2-level converter. This means that the lower slope of the carrier wave for the multi-level converter may lead to several crosses with the voltage reference in this area.
Application of conventional DBPWM in a cell based converter may lead to a lot of additional switching.
Massoud also mentions another way of modulation, modified sinusoidal PWM, where the carrier is modified. Massoud also mentions that this type of modulation leads to a complex hardware implementation. This hardware implementation will get even more complex if applied to a situation with several time-shifted carriers as described above in relation to WO 2009/097063.
There is therefore still a need for an improvement in the field of control of cells in a voltage source converter, especially in order to reduce the switching losses.